1. Technical Field
The present invention relates to an apparatus for controlling electric discharge of battery blocks constituting a secondary battery, as well as to a capacitance equalization apparatus for preventing occurrence of capacitive variations between battery blocks.
2. Related Art
An electric vehicle which acquires vehicle drive force by means of an electric motor, such as a pure electric vehicle or a hybrid electric vehicle, is equipped with a secondary battery, and the electric motor is driven by means of electric power stored in the secondary battery. Such an electric vehicle has a regenerative braking function; namely, a function of causing the electric motor to act as a generator during braking of the vehicle and converting kinetic energy of the vehicle into electric energy, thereby applying a brake. The thus-converted electric energy is stored in a secondary battery and reused when acceleration is performed, and the like.
Since battery performance of the secondary battery is deteriorated when the secondary battery is excessively discharged or charged, the state of charge (SOC) of the secondary battery must be grasped, to thus adjust discharging and charging operation.
Incidentally, variations occasionally arise in battery blocks in terms of battery performance depending on manufacturing processes or subsequent service conditions of the secondary battery, which in turn induces variations in the charging state of battery blocks constituting the secondary battery. In the secondary battery built by series connection of such battery blocks, when discharging and charging of the battery blocks are repeated, battery blocks having small capacitance will become excessively discharged, or battery blocks having large capacitance will become excessively charged, whereupon variations in the state of charge of battery blocks increase. Consequently, the range of capacitance within which the battery blocks can be used becomes narrower. Specifically, the life of the secondary battery decreases apparently. It is important to reduce and equalize the capacitive variations between the battery blocks constituting the secondary battery.
2005-328642 A describes a technique for charging, through a capacitance equalization sequence, battery blocks of low voltages with electric charges stored in capacitive means when the voltage of the battery blocks is measured by means of a voltage measurement sequence, thereby reducing capacitive variations between battery blocks.
2005-333717 A describes a technique for reducing capacitive variations between battery blocks by means of connecting discharge devices to respective battery blocks, measuring voltages of the respective battery blocks with voltage measurement circuits, and activating the discharge device of the battery block of high voltage, to thus cause the battery block to perform electric discharge.
2005-151683 A describes a technique for reducing charging of a battery at least when the voltage of the battery has come to a predetermined level or more, thereby uniformly charging the battery.
By the way, in a conceivable system, a discharge device and a voltage measurement device are connected in parallel to both terminals of each battery block, and a battery block which is an object of electric discharge is located in accordance with terminal voltages of the respective battery blocks measured by the voltage measurement device. The located battery block is subjected to electric discharge by means of the discharge device, thereby preventing capacitive variations in battery blocks. In such a system, for instance, when the voltage measurement device measures a terminal voltage at a predetermined measurement period while at least one battery block is taken as a unit and when the discharge device is activated concurrently, there is a potential of one battery block being simultaneously subjected to voltage measurement and electric discharge. In this case, the voltage measurement device and the discharge device are connected in parallel, and hence the voltage measured by the voltage measurement device is divided by the discharge device. Specifically, an error arises in the terminal voltage of the battery block measured by the voltage measurement device according to the case where the discharge device is not performing electric discharge and where the discharge device is performing electric discharge. Accordingly, in such a system, when capacitive variations between battery blocks are controlled in accordance with the terminal voltage of the battery block measured by the voltage measurement device, it may arise the case where capacitive variations between battery blocks cannot be reduced according to the operating state of the discharge device.